1. Field of Invention
The present invention relates to an optical pick-up device for reading and writing information from and to an optical disc such as a CD. More particularly, the invention relates to an improvement in arrangement of a tracking coil and a magnet in the optical pick-up device. Furthermore, the invention relates to an improvement in a magnetic spring for holding a lens holder having an objective lens in a magnetic neutral position.
2. Related Art
As shown in FIGS. 10 and 11, an optical pick-up device 900 comprises a driven portion 904 rotatably supported on a casing 901 around a central shaft 902 and including an optical system having an objective lens 903, a focus coil 905 and a tracking coil 906 which are attached to the driven portion 904, and a magnet 907 fixed to the casing 901 opposite to each tracking coil 906. A part of the casing 901 is provided as a yoke on the inside of the driven portion 904 and forms a parallel magnetic flux together with the magnet 907.
Furthermore, the tracking coil 906 is wound like a rectangle on the side surface of the driven portion 904 and is provided such that one side (hereinafter referred to as a driving side) 908 along the center 902 is positioned in the parallel magnetic flux. In consideration of a reduction in the size of the optical pick-up device 900, the tracking coils 906 are provided on the opposite side of the objective lens 903 with the central shaft 902 interposed therebetween.
In the optical pick-up device 900, the tracking coil 906 is conducted so that force for rotating the driven portion 904 is generated. Thus, a tracking operation can be carried out.
In the optical pick-up device, however, each tracking coil 906 is provided asymmetrically with respect to the central shaft 902. Therefore, when one of the tracking coils 906 approaches the magnet 907 during a tracking operation, the other tracking coil 906 goes away from the magnet 907. In other words, the tracking coils 906 have different positional relationships with the magnet 907 during the tracking operation, respectively. Consequently, the tracking coil 906 is varied on a magnetic circuit basis.
In addition, an opposite side 909 to be conducted in a reverse direction to the driving side 908 is sometimes affected by a magnetic field leakage from the magnet 907. For this reason, a phase disturbance is apt to be caused in a high-frequency region as shown in FIG. 12.
In the optical pick-up device 900, moreover, a magnetic spring for holding the driven portion 904 in the magnetic neutral position can be constituted by the magnet 907 and a magnetic piece 910 opposed to a region in which the magnetic field strength of the magnet 907 is changed.
The magnet 907 is magnetized uniformly. In general, therefore, it is necessary to provide the magnetic piece 910 in a position opposed to the outer peripheral edge of the magnet 907.
In many cases, however, the magnet has a large dimension and a space for attaching the magnetic piece cannot be kept in a position opposed to the outer peripheral edge. In particular, in the case in which the magnetic piece is to be provided on the outer peripheral edge of the magnet in a focusing direction, the magnetic piece should be provided in the position opposed to the outer peripheral edge of the magnet on the opposite side of an optical recording medium because there is no space on the optical recording medium side. As a result, the thickness of the optical pick-up device is determined depending on a dimension in the focusing direction occupied by the magnet and the magnetic piece opposed to the outer peripheral edge, which is not preferable for a reduction in the thickness of the device.
It is an object of the invention to provide an optical pick-up device which prevents a positional relationship with a magnet from being varied for each tracking coil during a tracking operation and prevents a side opposed to a driving side of the tracking coil from being affected by a magnetic field leakage.
It is another object of the invention to provide a magnetic spring for holding a lens holder in a magnetic neutral position without preventing a reduction in a thickness.